The purpose of this multi-center, randomized, double-blind, parallel-group, 26 week study is to compare the efficacy and safety of two different dose levels of Amantadine Extended Release Tablets to placebo for the treatment of levodopa induced dyskinesia in patients with Parkinson's disease.

The dose will be given once a day in the morning so that amantadine concentrations are maintained throughout the day for treating the levodopa induced dyskinesia, but will be lower during the night, potentially reducing the negative impact of amantadine on sleep.

This is a study to assess the efficacy and safety of oral Istradefylline (KW-6002) in patients with moderate to severe Parkinson's Disease. While on this study, participants will continue to take their usual, prescribed, stable regimen of Levodopa/Carbidopa or Levodopa/Benserazide therapy plus adjunct Parkinson's medications.

Patients will be randomized 1:1:1 to receive either Istradefylline 20 mg per day, or Istradefylline 40 mg per day or an equivalent placebo.

Patients will be treated for a 12 week period to demonstrate the effectiveness of Istradefylline in improving Parkinson's disease symptoms (referred to as improvement in patient OFF time) and that Istradefylline has an acceptable safety profile in this group.

Patients who meet the following criteria could be eligible to participate: